The purpose of this project is to biochemically and immunologically characterize type C RNA viruses in order to understand the mechanisms of oncogenesis by these viruses. Another goal of this research is to determine whether or not these viruses play a role in the etiology of human cancers. Studies currently in progress are the following: 1) Biochemical characterization of ts mutants of MuLV; 2) Studies of the cellular control of endogenous viral gene expression in normal and malignant cells; 3) Search for the presence of type C viral gene products in human tumors; 4) Characterization of new type C virus isolates.